In the Laboratory
W
The Glyoxal Clock Reaction Julie B. Ealy,* Alexandra Rodriguez Negron, Jessica Stephens, and Rebecca Stauffer Department of Chemistry, Pennsylvania State University, Lehigh Valley Campus, Fogelsville, PA 18051; *
[email protected] Stanley D. Furrow Department of Chemistry, Pennsylvania State University, Reading, PA 19610
Clock reactions manifest their dramatic effect by a change in color after a predetermined period of time through varying the quantities of reactants. The formaldehyde–hydrogen sulfite∙sulfite clock is one of the simplest reagent combinations for a clock reaction. Throughout the years formaldehyde has also been used for student experiments and teacher demonstrations (1–5). In the formaldehyde–hydrogen sulfite∙sulfite clock, a sudden change in pH of the reaction mixture permits monitoring of the rate of the reaction with an indicator such as phenolphthalein (6). In 1929 Carl Wagner (7) proposed a mechanism for a formaldehyde clock that Cassen (2) used for his interpretation. Cassen suggested that even though the reaction is quite complex, it could be used to simulate the method of initial rates as applied to a bimolecular reaction (2). But the orders he suggested for formaldehyde, 1, and metabisulfite∙sulfite, ‒1, are inconsistent with a bimolecular reaction. A number of authors (2, 4, 6, 8, 9) have suggested a mechanism for the formaldehyde clock, though there is disagreement among them. In 1990, the late Miles Pickering suggested trying glyoxal as a possible substitute for formaldehyde in different reactions (10) as formaldehyde and glyoxal are structurally similar. One of the reactions was a glyoxal clock reaction (11, 12). In addition, adaptation of the glyoxal clock reaction to the use of a calculatorbased data acquisition system such as a LabPro (13) interfaced with a pH sensor was tested. Other kinetic reactions (14, 15) have been adapted to another calculator-based data acquisition system, the Calculator Based Laboratory (CBL) (16), a device very similar to the LabPro. Universities are also using the CBL for general chemistry labs (17–20) and undergraduate research (21). The LabPro, interfaced with a pH sensor, is easy to use and versatile and in this reaction provides a more reliable measure of the reaction time than simply timing the change in color, which has been the traditional method for clock reactions.
8.1, the endpoint is missed with phenolphthalein, which is the indicator used traditionally for the formaldehyde clock. Phenol red is used instead; however, since it changes from yellow to red, it is not as easy to time the color change. Though a pH change is monitored in the reaction with a sensor, students enjoy the color change associated with an indicator. The equivalence points, therefore, are determined both potentiometrically and with an acid–base indicator. The procedure was repeated for varying quantities of the two solutions. The reactants were mixed and the pH of the mixture was recorded as a function of time. An example of one of the graphs based on student lab data with the Graphical Analysis software program (15) is shown in Figure 1. Graphical Analysis links a graphing calculator directly to a computer using a graph link cable. The data are then transferred into Graphical Analysis and graphed automatically. A pedagogically useful method of determining the equivalence point and end of the reaction is illustrated in Figure 1 (22): tangents are drawn to the beginning and ending sections of the graph and then the two tangents are connected with a perpendicular bisector (see details in the Supplemental MaterialW). The equivalence point could also be obtained from a first derivative of the appropriate columns of data. This can be done easily with Graphical Analysis. The Reactions Sulfite and hydrogen sulfite are present in this clock system, so initially the system is buffered and the pH changes slowly. As the hydrogen sulfite is used the pH slowly rises until [HSO3−] is very low; at this point the pH rises rapidly since the buffering capacity has been exceeded. It is for this reason the clock reaction can be monitored with a pH sensor. In the clock reaction, the pH starts near 6, and by the time the hydrogen sulfite is mostly
Experimental Section
9.0
Two solutions are utilized in the experiment: a glyoxal solution1 and a sodium metabisulfite∙sodium sulfite solution with Na2EDTA⋅2H2O (details in the Supplemental MaterialW ). Phenol red is used as an indicator. The EDTA serves as a stabilizing agent against air oxidation of sulfite to sulfate by sequestering non-alkali metals that can catalyze the air oxidation. As an aside, the metabisulfite hydrolyzes rapidly to two moles of hydrogen sulfite and the same results can be expected using 0.189 M NaHSO3 instead of 0.0947 M Na2S2O5. The following equation represents the hydrolysis
8.5
2
S2O5 (aq) H2O(l)
2HSO3 (aq)
Phenol red is the indicator suggested for the glyoxal clock. Since the pH range for the glyoxal clock is approximately 5.5 to
pH 7.43
8.0 7.5 7.0 6.5 6.0
0 initial time end of reaction
10
20
30
40
50
60
70
Time / s Total reaction time = 35.0 s − 17.7 s = 20.3 s
Figure 1. pH versus time, 0.00945 M glyoxal; 0.00112 M metabisulfite, 0.0000280 M sulfite (see the Supplemental MaterialW for calculation details).
www.JCE.DivCHED.org • Vol. 84 No. 12 December 2007 • Journal of Chemical Education 1965
In the Laboratory −0.08
y = mx + b m = −0.0000837 b = −0.00602 correlation = −0.987
−0.002
−0.004
−0.006
−0.008
ln (−%[glyoxal] /% time)
% [HSO3ź] / (mol/L)
0.000
−0.010
0
15
30
45
60
75
90
105
−0.09
y = mx + b m = 0.990 b = −5.37 correlation = 0.994
−0.10
−0.11
−0.12
120
−6.0
Time / s
−5.5
−5.0
−4.5
−4.0
−3.5
−3.0
ln [glyoxal]0
Figure 2. Δ[HSO3−] versus time.
Figure 3. ln({Δ[glyoxal])/Δtime) versus ln[glyoxal]0.
consumed, the pH is between 8 and 9. The net reaction1 is
To determine the order of glyoxal, ln(‒Δ[glyoxal]∙Δtime) versus ln[glyoxal]0, the intial glyoxal concentration, was plotted. The order is found from the slope of the graph. The data in Figure 3 show a slope of 0.990 and thus an order of l. This is the same as the slope reported for formaldehyde by Cassen (2).
(HO)2CHCH(OH)2(aq) 2HSO3 (aq) O3SCH(OH)CH(OH)SO3(aq) 2H2O(l) Laboratory Time Needed The experiment can be completed in three hours. It is also suggested that one additional three-hour lab period be devoted to analysis of the data where students could work together. Hazards Glyoxal solution (40%) is moderately irritating to skin and mucous membranes. Phenol red, sodium metabisulfite, sodium sulfite, and EDTA solids may be harmful by inhalation, ingestion, or skin absorption. Analysis of the Data As mentioned previously, the actual species in solution is hydrogen sulfite, which is twice the molar concentration of metabisulfite, so hydrogen sulfite appears in the rate expression. The rate expression for the clock reaction is m n rate k For the series where [glyoxal] is a constant, the rate equation can be simplified to rate = k′[hydrogen sulfite]n, where k′ = k[glyoxal]m. A plot of Δ[HSO3−] versus time for that series (Figure 2) is a straight line, indicating the order of hydrogen sulfite is zero, so [HSO3−]n = 1. The resulting rate expression is, therefore, rate k
m
It is assumed in this analysis that any change in the initial concentration of EDTA does not affect the pH change. The concentration of EDTA present in each run is proportional to the concentration of metabisulfite∙sulfite so that the initial pH is the same for each run. EDTA will contribute a very minor degree to the buffering effect of hydrogen sulfite∙sulfite; being proportional, however, the effect should be the same in all the solutions.
Conclusion The glyoxal clock presents an opportunity for students in general chemistry to study the kinetics of a clock reaction. The chemicals are safe to use in a laboratory and the order of the reactants, first order in glyoxal and zero order in hydrogen sulfite provide interesting results. The clock reaction could also be extended to physical chemistry where a more detailed study of the order of hydrogen sulfite could be investigated, and the rate constants determined. Discussion of the mechanism and utilization of UV–vis spectrophotometry could provide a more in depth challenge to the glyoxal clock. A stopwatch could be used to time the clock and the average of trials could be utilized to plot graphs to obtain the apparent order of both reactants. However, the experiment proves interesting when carried out with the use of a pH sensor linked to a LabPro or CBL. Students receive reinforcement visually of the time with a change in the color of the indicator and also through obtaining the time from the method of tangents or from a first derivative versus time. Acknowledgment We would like to thank Roger Egolf, Penn State Lehigh Valley, for his help with the organic chemistry of the mechanism. WSupplemental
Material
Handouts for the students and notes for the instructors are available in this issue of JCE Online. Note 1. In an aqueous solution, glyoxal is mostly dihydrated, that is, it exists as (HO)2CHCH(OH)2.
1966 Journal of Chemical Education • Vol. 84 No. 12 December 2007 • www.JCE.DivCHED.org
In the Laboratory
Literature Cited 1. Barrett, R. L. J. Chem. Educ. 1955, 32, 78–79. 2. Cassen, T. J. Chem. Educ. 1976, 53, 197–198. 3. Fortman, J. J.; Schreler, J. A. J. Chem. Educ. 1991, 68, 324. 4. Jones, P.; Oldham, K. B. J. Chem. Educ. 1963, 40, 366–367. 5. Alyea, H. J. Chem. Educ. 1958, 35, A215. 6. Warneck, P. J. Chem. Educ. 1989, 66, 334–335. 7. Wagner, C. Berichte 1929, 62, 2873–2877. 8. Boyce, S. D.; Hoffman, M. R. J. Phys. Chem. 1984, 88, 4740–4746. 9. Burnett, M. G. J. Chem. Educ. 1982, 59, 160–162. 10. Ealy, J. B. Science Teacher 1991, 58 (4), 26–27. 11. Ealy, J. B. Chem 13 News 1990, 195, 9. 12. Ealy, J. B.; Ealy, J. L., Jr. Visualizing Chemistry: Investigations for Teachers; American Chemical Society: Washington, DC, 1995; pp 195–198. 13. Vernier Home Page. http://www.vernier.com (accessed Sep 2007).
14. Cortes-Figueroa, J. E.; Moore, D. A. J. Chem. Educ. 1999, 76, 635–638. 15. Roser, C. E.; McCluskey, C. L. J. Chem. Educ. 1999, 76, 1514– 1515. 16. Texas Instruments-New Education Home Page. http://education. ti.com (accessed Sep 2007). 17. Chau, F. T.; Mok, K. W. J. Autom. Chem. 1992, 14 (3), 79– 83. 18. NSF grant. http://www.mathmachines.net/nsfgrant.htm (accessed Sep 2007). 19. Freeman, D. http://www.chem.sc.edu/news/USCChemist/ spring1999/CAPA.html (accessed Sep 2007). 20. Instrumentation. http://home.sjfc.edu/chemistry/Chemistry_Instrumentation.htm (accessed Sep 2007). 21. Sales, C. L.; Ragan, N. M.; Murphy, M. K. J. Chem. Educ. 2001, 78, 694–696. 22. Chemtopics. http://www.chemtopics.com/aplab/diprotic.pdf (accessed Sep 2007).
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